EP 1 414 275 B1 describes an electrical heater with a tubular housing, which surrounds a heating insert. The heating insert includes a PTC thermistor, which is also referred to as a PTC heating element, since the PTC thermistor has a positive temperature coefficient (PTC=positive temperature coefficient). Thus, the electrical resistance of the PTC thermistor increases with increasing temperature such that it limits the temperature to the top in a self-regulating manner. The heating insert also includes two heat transfer means, wherein the PTC thermistor is disposed between the two heat transfer means. Viewed in radial direction the PTC thermistor is disposed between the two heat transfer means. The heat transfer means transfer the thermal heat of the PTC thermistor to the housing. Each of the two heat transfer means has a flat basic part on its side facing the PTC thermistor, on which two bent legs are disposed. The legs abut an inner wall of the housing in the manner of a flexible spring with the heating insert inserted in the housing. By the resilience of the legs, good abutment of the heat transfer means on the housing and thus good heat transfer from the PTC thermistor to the housing is provided.
In an electrical heater described in DE 102 58 257 A1, the abutment of the heating insert on the inner wall of the housing is accomplished by first inserting the heating insert into the housing—rectangular in cross-section—and then compressing the housing. This subsequent compression of opposing sides of the housing is also referred to as lateral press-fitting. Herein, the housing is made of an aluminum alloy.
U.S. Pat. No. 6,180,930 B1 relates to an electrical heater with a heating insert, which only includes a PTC thermistor and two contact electrodes in a common electrically isolating envelope. This heating insert is inserted in a housing of aluminum or copper—also rectangular in cross-section. The two opposing side walls of the housing are moved towards each other by compressing the housing in order to clamp the heating insert between them. The two side walls not pressurized, which may be concave or convex, are formed in deformable manner in order to be able to accommodate the loading upon compressing the housing.
However, limitations are imposed on the above described possibilities of placing the heating insert in contact with the housing as close as possible. In a comparatively long tubular housing and correspondingly comparatively long heating insert, it is difficult to insert the heating insert into the housing, if it is resiliently supported on the inner side of the housing. By subsequently compressing opposing sides of the housing, a completely satisfactory heat transfer also cannot always be ensured.
Therefore, it is the object of the present invention to provide an electrical heater and a method of the initially mentioned type, in which or by which a particularly good heat transfer from the heating insert to the housing is present even with a comparatively long housing.
This object is solved by an electrical heater having the features of claim 1 and by a method having the features of claim 11. Advantageous configurations with convenlent developments of the invention are specified in the dependent claims.